DTM.5:2 - Problem

An overall mean mixes changes within groups with changes in the groups’ contributions to what continues. Calling either component “group success” leaves its unit unclear: more copied methods, more learners, survival of the same organization, or more successor organizations.

The same arithmetic can also arise without selection. Reconstruction, unequal exposure, recruitment or a changed measurement can alter the mean. A useful account must therefore distinguish the bookkeeping identity from the causal explanation.